Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
1998-10-15
2001-12-25
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S308000, C430S281100, C430S270100, C430S271100, C522S071000, C522S074000, C522S110000, C522S135000
Reexamination Certificate
active
06333137
ABSTRACT:
This invention concerns a screen printing stencil, and more particularly a photocurable composition for the production of a screen printing stencil.
In the screen printing process ink is forced through a mesh of a polymer (usually polyester) or of stainless steel stretched over a wooden or metal frame, using a squeegee. In the areas of mesh where printing is not to be effected the mesh is covered with a negative of the print design, known as a stencil. The stencil is composed of a material impervious to the ink to be used for the printing and is usually formed by imagewise exposure of the areas to form the stencil and subsequent aqueous development.
Many methods have been proposed for the production of screen printing stencils. Stencils can be prepared either independently of the screen, by the indirect method, or on the screen itself by the direct method.
In the indirect method, a photosensitive layer of emulsion is coated on to a temporary support sheet, usually a transparent polyester sheet, and following exposure to actinic light the layer of emulsion is developed on the support sheet, using water to remove unexposed areas of the photosensitive layer. The wet stencil on its support is then adhered to the screen by the application of gentle, even pressure. After drying, the support sheet is removed, leaving the stencil on the screen mesh ready for printing.
In the direct method, a photosensitive emulsion is coated on to a mesh and dried to give a continuous, even film. A stencil is then produced by imaging the coating directly with a laser, or photographically through a line or half tone positive, and then developing an image by removing unexposed areas of the film with water.
A further technique, the so-called ‘capillary film’ method, involves the application of a photosensitive layer to a polyester sheet, which is then transferred to a mesh prior to imaging. Imaging and development are then effected as described for the direct method.
Combinations of emulsions and films are also possible in which a direct emulsion is used to adhere a film to a mesh. Development of the stencil is then as with the direct method.
The photosensitive coatings for all these techniques are polymer based, and formulated to achieve chemical, thermal and mechanical resistance in use and during manufacture, dimensional stability, flexibility, maintenance of light stability and impermeability to liquids.
Compositions used to prepare screen printing stencils are in general based on water soluble colloids, usually polyvinyl alcohols, which act as binder resins. The compositions are such that areas of the coatings irradiated by actinic light become insoluble in water and remain on the screen after development, while areas protected from actinic light retain their water solubility and are easily washed away with water.
Image development with water necessitates rendering a coating of a water soluble colloid binder insoluble to water as a consequence of exposure to actinic light. This can be achieved either by crosslinking the colloid or by forming a secondary polymer matrix within the coating structure, thereby preventing redispersion.
A secondary polymer matrix insoluble in water is formed by photopolymerising compositions containing photopolymerisable monomers and/or oligomers possessing light sensitive groups, or to which a photoinitiator system has been added. Oligomers are preferred because the rate of insolubilisation of the compositions on exposure is usually more rapid, presumably due to a rapid establishment of a network polymer structure and also because of their lower rate of evaporation on drying. Unfortunately, stencils made entirely of secondary polymer matrix cannot be easily removed after printing as the matrix is not easily degraded.
Crosslinking of colloids can be achieved by a number of methods using ultra-violet or visible radiation. Colloidal polymer molecules having reactive groups along the polymer chain can crosslink with other polymer molecules via such groups. Alternatively a photochemically activated crosslinking agent can be used. The resultant increase in molecular weight of the colloid reduces its solubility in water.
Important requirements for colloidal binder resins for producing screen printing stencils are solvent resistance, decoatability and water solubility. Polyvinyl alcohol derivatives meet all these requirements as well as having good mechanical strength and heat resistance. Furthermore, the removal of polyvinyl alcohol based coatings following printing is simply achieved by selectively oxidising the 1,2-diol groups on the polymer chain, for example, by applying a sodium metaperiodate solution. This is known as the decoating process.
The water solubility of polyvinyl alcohol derivatives is related both to their molecular weight and to their degree of hydrolysis. In general, they are rendered water soluble when at least 70% of the acetate groups of the precursor polyvinyl acetate are hydrolysed to hydroxy groups. Totally hydrolysed polyvinyl alcohols are only slightly soluble in cold water as a result of strong intermolecular hydrogen bonding. Lower molecular weight grades of polyvinyl alcohol tend to be more soluble in water than higher molecular weight grades.
The use of polyvinyl alcohol in photocrosslinkable compositions for producing screen printing stencils is well known in the art. Original systems were photosensitised by adding a dichromate compound or polymeric diazonium salt. An improved method of crosslinking polyvinyl alcohol is described in British Patent specifications 2030575, 2288402, 2288403 and 2226564, and European Patents 0373537 and 0555070, in which pendant styryl pyridinium groups appended to the polyvinyl alcohol dimerise under the influence of actinic light. The resultant increase in the molecular weight of the polymer renders the coatings of the polymer insoluble in water, enabling stencil development to take place.
For environmental reasons water based screen printing inks are becoming increasingly popular and the need for stencils having good water resistance is increasing. If stencils do not have adequate water resistance they will swell, soften and be mechanically degraded by the printing processes, resulting in early stencil breakdown. Water based inks may also contain oligomers, monomers, surfactants and solvents, which increase the tendency of the stencil to soften and swell.
The problem with resistance to water of water based inks is more marked with stencils that have been photographically imaged using a projection system where actinic light is projected on to a distant screen through a positive smaller than the screen. Due to lower UV light intensities, the crosslinking is less effective than with direct exposure to actinic light through a full sized positive. Such exposure systems are now being more widely used. Furthermore, because of the low intensities, emulsions based on diazo sensitisers have too long an exposure time for practical use and, whilst dichromate sensitised emulsions have a shorter exposure time, they are not used for health reasons. Emulsions based on diazo and dichromate sensitisers need to have the sensitiser added just prior to use and have a limited life once sensitised. Emulsions based on styryl pyridinium groups are supplied presensitised and have a long useful life. Emulsions based on styryl pyridinium groups have a short enough exposure time when used on a projection system, are convenient to use as they do not need to be sensitised prior to use, and have a good useful life. Unfortunately, they do not have very good water resistance.
Skilled persons can formulate stencil making compositions to form stencils that will be inherently resistant to water based inks. However, the durability of such stencils is often inadequate for the print run required, especially when exposed with a projection system. Hydrophobic polymers can be introduced into the stencil making composition to produce stencils having improved water resistance; however, solvents are often required to clean water based inks from the stencils,
Collins Nicholas R.
Dickinson Peter
Baxter Janet
Lee Sim J.
Nixon & Vanderhye P.C.
Sericol Limited
LandOfFree
Screen printing stencil does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Screen printing stencil, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Screen printing stencil will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2597229